1. Field of the Invention
The present invention relates to a printing roller used as, for example, an ink distributing roller, an ink form roller, a rider or a fountain roller included in an ink mechanism for a printer, a metering roller replacing an anilox roller, an ink transfer roller, a metering roller or an ink fountain roller included in a keyless inking system, or a fountain roller, a distributing roller, a metering roller or an applicator roller included in a coating machine of a paint or an adhesive, as well as to a method of manufacturing the same.
2. Description of the Related Art
It is widely known to the art to use a rubber roller for the control of an ink amount, and it was customary in the past to apply the particular idea to an ink distributing roller. For example, a rubber roller is used for forming an ink transfer roller included in a keyless inking system constructed not to use a finger grip for controlling the ink amount. The particular ink transfer roller is used mainly in a keyless offset rotary press for the news paper printing.
FIG. 1 schematically shows a conventional keyless offset rotary press. As shown in the drawing, the rotary press includes an ink fountain roller 1 which is rotated at a low speed to draw up an ink. An ink transfer roller 2 serving to transfer the ink supplied from the ink transfer roller 1 is arranged adjacent to the roller 1. It should be noted that the roller 2 also acts as an ink metering roller which constitutes a key point in a keyless printing machine. The ink received by the roller 2 is supplied to a doctor roller 3 which is arranged adjacent to the roller 2. A rider roller 4 serving to make uniform the thickness of the ink layer formed on the doctor roller 3 is arranged adjacent to the doctor roller 3. The ink supplied from the doctor roller 3 is transferred onto a printing paper sheet via form rollers 5 and a printing cylinder 6. Further, a steel blade 7 made of a Swedish steel is in contact under pressure with the doctor roller 3.
In the keyless offset rotary press of the construction described above, the ink is transferred from the fountain roller 1 onto the ink transfer roller 2. The main factors determining the quality of the printing such as the concentration of the printed image and uniformity of the printing are determined in this step. The ink is further transferred from the ink transfer roller 2 onto the doctor roller 3 and, then, the ink layer is made uniform by the rider roller 4. Further, the ink is transferred via the inked rollers 5 onto the plate cylinder 6. It should be noted that the ink, which has not been transferred onto the plate cylinder 6 so as to be left on the inked rollers 5, is scraped off the form rollers 5 by the steel blade 7 which is in contact under pressure with the doctor roller 3. It follows that a predetermined amount of ink is kept supplied onto the plate cylinder 6.
In the conventional keyless offset rotary press, the ink fountain roller 1 is rotated at a low speed to draw up an ink. On the other hand, the ink transfer roller 2 abutting against the roller 1 is rotated at a high speed so as to receive ink from the roller 1. The amount of the ink received by the ink transfer roller 2 is greatly dependent on the coarseness and shape on the surface of the roller 2. It should be noted that the ink transfer roller 2 abuts against the ink fountain roller 1 with a nip width of 5 to 10 mm. In addition, the peripheral speed of the roller 2 is 50 times as much as that of the roller 1. In other words, the ink transfer roller 2 is operated under very severe conditions.
It is certainly possible to control easily the ink amount by increasing the nip pressure. In this case, however, the heat generation is increased because of the high pressure and the large difference in the peripheral speed between the rollers 1 and 2. At the same time, the surface of the ink transfer roller 2 is severely abraded, leading to a marked reduction in the life of the roller 2. On the contrary, if the nip pressure is lowered, it is certainly possible to suppress the heat generation and abrasion of the roller surface. In this case, however, it is difficult to reproduce the pattern of the roller surface satisfactorily. In addition, problems are brought about in terms of misting of ink and uneven ink amount.
It is also important to pay attentions to the shape on the surface of the ink transfer roller. If the projections and recesses forming the surface coarseness of the roller are formed with a small pitch, the ink can be transferred uniformly, making it possible to prevent a nonuniform printing. However, the depth of the recess is decreased and, thus, the amount of the ink which is transferred is diminished, leading to emulsification of the ink and to a short life of the roller. On the contrary, if the projections and recesses are formed with a large pitch, the depth of the recess can be increased, leading to a large amount of the ink which is received. In this case, however, the transferred ink amount is rendered nonuniform. As a matter of fact, the nonuniformity is so large that it is impossible to eliminate the nonuniformity even if the transferred ink is kneaded with an ink distributing roller (called a rider roller), with the result that the thickness of the ink transferred onto a paper sheet or the like is rendered uneven.
In order to achieve a required ink thickness and to prevent emulsification of ink, the projection-recess pitch is set at such a large value as 1 to 2 mm in the conventional roller. Also, the surface coarseness is so large as not to be measured accurately. In other words, the conventional roller is used at the sacrifice of the quality of the solid coverage.
It should also be noted that it is very difficult to ensure the required surface coarseness falling within a desired range by the conventional grinding technique because the surface coarseness on the surface of the conventional roller is very small. In other words, the surface coarseness is rendered uneven in the conventional technique, leading to unevenness in the quality of the printed surface, in the life, in the emulsification of ink, etc. In order to overcome these difficulties, vigorous studies have been made in an attempt to develop a technique of engraving the surface of a rubber roller with a laser cutting technique. Specifically, prepared were three kinds of rollers having the surfaces engraved to form lattice patterns each including a large number of diamond configurations as shown in FIGS. 2A to 2C. In the roller shown in FIG. 2A, each diamond configuration includes two diagonally facing corners each having an angle of 60.degree.. These diamond configurations are of a projection type. In other words, these diamond configurations are separated from each other by a groove. In the roller shown in FIG. 2B, each diamond configuration includes two diagonally facing corners each having an angle of 120.degree.. These diamond configurations are also of a projection type. Further, in the roller shown in FIG. 2C, each diamond configuration includes two diagonally facing corners each having an angle of 90.degree.. These diamond configurations are of a depression type. In other words, the depressions are formed separately from each other, failing to form a continuous groove.
Printing was actually performed by mounting each of these three kinds of rollers in place of the ink transfer roller 2 included in the keyless offset rotary press shown in FIG. 1. It has been found that, in the case of the depression type shown in FIG. 2C, the lattice pattern is reproduced on the printed paper sheet or the like. Also, a severe ink misting has been found to take place. Further, the ink concentration on the printed paper sheet or the like has been found to be low. When it comes to the projection type as shown in FIG. 2A or 2B, reproduction of the lattice pattern has been found to be negligibly small. The ink misting has also been found to be low. It should be noted that, in the projection type, the ink is carried through the continuous groove defining the diamond configurations of the projection type. In other words, the ink fluidity is ensured in the projection type so as to suppress the lattice pattern reproduction and the ink misting as pointed out above. In the case of the depression type, however, the diamond configurations form independent cells. In other words, a continuous groove is not formed in the lattice pattern of depression type shown in FIG. 2C, giving rise to the serious problems pointed out above.
However, it is difficult to control the engraving depth by the laser cutting technique, resulting in failure to form a desired pattern on the roller surface and in failure to prevent the pattern from being reproduced on the printed paper sheet or the like. Further, the laser cutting technique is costly and, thus, is unsuitable for the practical application. On the other hand, an ink transfer roller having the surface treated by the conventional grinding method utilizing a grinder or a cutter certainly belongs to a projection type. In this roller, however, the pattern formed on the surface is parallel with the axis of the roller and is in the shape of a wavy projection pattern, with the result that the roller resembles the separated depression type shown in FIG. 2C in its ink transfer function. To be more specific, the conventional roller prepared by employing the conventional grinding technique, which certainly permits increasing the amount of ink to be transferred, lacks continuity so as to allow the ink to be transferred intermittently, leading to a poor ink fluidity and, thus, to serious problems such as emulsification of ink.
The conventional ink transfer roller also includes a roller having the surface grooved in a direction close to its circumferential direction. However, the conventional roller of this type is incapable of preventing the pattern on the roller surface from being reproduced on the printed paper sheet. It is also impossible for the roller to supply ink in an amount required for the ink transfer roller. If it is intended to suppress reproduction of the surface pattern on the printed paper sheet, it is unavoidable to diminish the pitch of the grooved pattern on the surface of the roller. However, the amount of ink to be transferred is further decreased, if the pitch of the grooved pattern is diminished. It is certainly possible to increase the amount of ink to be transferred, if the groove formed on the roller surface extends in a direction making a large angle with the axis of the roller. In this case, however, the grooved pattern on the roller surface is more likely to be reproduced on the printed paper sheet.
When it comes to a grooved roller for the coating of a paint or an adhesive, grooves are formed on the surface of a roller in a screen ruling of 10 to 30 lines/inch such that these grooves extend in the circumferential direction. A paint or adhesive is stored in these grooves so as to permit forming a coated film of a predetermined thickness. In the case of coating an adhesive, an uneven coating does not take place as a serious problem and, thus, the roller for this purpose differs from the ink transfer roller included in a keyless offset rotary press. However, roller for the coating of an adhesive has a short life because of abrasion of the rubber roll, leading to a high cost. In general, a gravure roller system using an anilox roller is employed for the coating of a film with a paint. In this case, a steel blade is brought into contact under pressure with the roller surface so as to remove an excess paint and, thus, to obtain a coated film of a predetermined thickness. Since a steel blade is brought into contact under pressure with the roller surface, the cost, life, etc. of the anilox roller provide serious problems to be solved in the future.
As described above, a metering capability of a suitable amount of an ink is required for a metering roller included in a keyless offset rotary press. It is also required for the metering roller to be capable of preventing the pattern on the surface of an ink transfer roller from being reproduced on the printed paper sheet, to be capable of achieving a high printing stability (or low in unevenness of the printed paper sheet), and to be capable of reducing the cost. This is also the case with the metering roller included in a coating machine.